Patch for https://www.argyllcms.com/ dispcal to add 20 point grayscale RGB calibration (can be used for 10 point also).

dispcal.patch

--- Argyll_V2.1.1/spectro/dispcal.c	2019-04-29 12:33:26.000000000 -0400
+++ Argyll_V2.1.1_20point/spectro/dispcal.c	2019-05-26 07:40:16.662940045 -0400
@@ -3105,7 +3105,7 @@
 			int c;
 
 			/* Print the menue of adjustments */
-			printf("\nPress 1 .. 7\n");
+			printf("\nPress 1 .. 7 or a .. u\n");
 			printf("1) Black level (CRT: Offset/Brightness)\n");
 			printf("2) White point (Color temperature, R,G,B, Gain/Contrast)\n");
 			printf("3) White level (CRT: Gain/Contrast, LCD: Brightness/Backlight)\n");
@@ -3114,6 +3114,10 @@
 			printf("6) Measure and set ambient for viewing condition adjustment\n");
 			printf("7) Continue on to calibration\n");
 			printf("8) Exit\n");
+			printf("a) 0pct Grayscale point (R,G,B)\n");
+			for (int i = 98; i < 118; i++) {
+				printf("%c) %dpct Grayscale point (R,G,B)\n", (char) i, (i - 97) * 5);
+			}
 
 			empty_con_chars();
 			c = next_con_char();
@@ -3804,7 +3808,157 @@
 				printf("Exiting\n");
 				dr->del(dr);
 				exit(0);
-			}
+			} else {
+				if (c > 118 || c < 97) {
+					continue;
+				}
+				col tcols[3];
+				double lpoint = (((double) c - 97.0) * 5.0) / 100.0;
+				tcols[0].r = tcols[0].g = tcols[0].b = lpoint;
+				tcols[1].r = tcols[1].g = tcols[1].b = ((1.0 - lpoint) / 2.0) + lpoint;
+				tcols[2].r = tcols[2].g = tcols[2].b = 1.0;
+				int ff;
+				double tYxy[3];				/* Target white chromaticities */
+				icmXYZNumber tXYZ;			/* Target white as XYZ */
+				double tLab[3];				/* Target white as Lab or UCS */
+				double mgamma, tar1, dev1;
+				double Lab[3];				/* Last measured point Lab */
+		
+				printf("Doing some initial measurements\n");
+
+				if (rgbch == 0) {	/* Figure the RGB chromaticities */
+					col ccols[3] = {
+						{ 1.0, 0.0, 0.0 },
+						{ 0.0, 1.0, 0.0 },
+						{ 0.0, 0.0, 1.0 }
+					};
+					if ((rv = dr->read(dr, ccols, 3, 0, 0, 1, 0, instClamp)) != 0) {
+						dr->del(dr);
+						error("display read failed with '%s'\n",disprd_err(rv));
+					}
+					if (verb) {
+						printf("Red   = XYZ %6.3f %6.3f %6.3f\n",ccols[0].XYZ[0],
+						                       ccols[0].XYZ[1], ccols[0].XYZ[2]);
+						printf("Green = XYZ %6.3f %6.3f %6.3f\n",ccols[1].XYZ[0],
+						                       ccols[1].XYZ[1], ccols[1].XYZ[2]);
+						printf("Blue  = XYZ %6.3f %6.3f %6.3f\n",ccols[2].XYZ[0],
+						                       ccols[2].XYZ[1], ccols[2].XYZ[2]);
+					}
+					for (i = 0; i < 3; i++)
+						icmAry2Ary(rgbXYZ[i], ccols[i].XYZ);
+					rgbch = 1;
+				}
+				/* Do an initial set of readings to set 1% output mark */
+				if ((rv = dr->read(dr, tcols, 3, 0, 0, 1, 0, instClamp)) != 0) {
+					dr->del(dr);
+					error("display read failed with '%s'\n",disprd_err(rv));
+				}
+				if (verb) {
+					printf("Low = XYZ %6.4f %6.4f %6.4f\n",tcols[0].XYZ[0],
+					                                         tcols[0].XYZ[1], tcols[0].XYZ[2]);
+					printf("Med  = XYZ %6.3f %6.3f %6.3f\n",tcols[1].XYZ[0],
+					                                         tcols[1].XYZ[1], tcols[1].XYZ[2]);
+					printf("High = XYZ %6.3f %6.3f %6.3f\n",tcols[2].XYZ[0],
+					                                         tcols[2].XYZ[1], tcols[2].XYZ[2]);
+				}
+				
+				/* Advertised Gamma - Gross curve shape */
+				mgamma = pop_gamma(tcols[0].XYZ[1], tcols[1].XYZ[1], tcols[2].XYZ[1]);
+
+				dev1 = pow(lpoint, 1.0/mgamma);
+				tcols[1].r = tcols[1].g = tcols[1].b = dev1;
+
+				/* Figure out the target white chromaticity */
+				if (wpx > 0.0 || wpy > 0.0) {	/* xy coordinates */
+					tYxy[0] = 1.0;
+					tYxy[1] = wpx;
+					tYxy[2] = wpy;
+		
+				} else if (temp > 0.0) {		/* Daylight color temperature */
+					double XYZ[3];
+					if (planckian)
+						rv = icx_ill_sp2XYZ(XYZ, icxOT_default, NULL, icxIT_Ptemp, temp, NULL, 0);
+					else
+						rv = icx_ill_sp2XYZ(XYZ, icxOT_default, NULL, icxIT_Dtemp, temp, NULL, 0);
+					if (rv != 0)
+						error("Failed to compute XYZ of target color temperature %f\n",temp);
+					icmXYZ2Yxy(tYxy, XYZ);
+				} else {						/* Native white */
+					icmXYZ2Yxy(tYxy, tcols[2].XYZ);
+				}
+
+				printf("\nAdjust R,G & B offsets to get target x,y. Press space when done.\n");
+				printf("   Target x %.4f , y %.4f \n", tYxy[1],tYxy[2]);
+				for (ff = 0;; ff ^= 1) {
+					double Yxy[3];		/* Yxy of current reading */
+					double rgbdir[3];	/* Direction to adjust RGB */
+					double rgbxdir[3];	/* Biggest to move */
+					double bdir, terr;
+					int bx = 0;
+					
+					if ((rv = dr->read(dr, tcols, 1, 0, 0, 1, ' ', instClamp)) != 0) {
+						if (rv == 4)
+							break;			/* User is done with this adjustment */
+						dr->del(dr);
+						error("display read failed with '%s'\n",disprd_err(rv));
+					} 
+				
+
+					/* Compute numbers for black point error and direction */
+					icmYxy2XYZ(tLab, tYxy);
+					tLab[0] /= tLab[1];
+					tLab[2] /= tLab[1];
+					tLab[1] /= tLab[1];
+					icmAry2XYZ(tXYZ, tLab);				/* Lab white reference */
+					icmXYZ2Lab(&tXYZ, tLab, tLab);
+
+					icmXYZ2Yxy(Yxy, tcols[1].XYZ);
+					icmAry2Ary(Lab, tcols[1].XYZ);
+					Lab[0] /= Lab[1];
+					Lab[2] /= Lab[1];
+					Lab[1] /= Lab[1];
+					icmXYZ2Lab(&tXYZ, Lab, Lab);
+	
+					/* Compute dot products */
+					bdir = 0.0;
+					for (i = 0; i < 3; i++) {
+						double rgbLab[3];
+						
+						icmXYZ2Lab(&tXYZ, rgbLab, rgbXYZ[i]);
+						rgbdir[i] = (tLab[1] - Lab[1]) * (rgbLab[1] - Lab[1])
+						          + (tLab[2] - Lab[2]) * (rgbLab[2] - Lab[2]);
+						rgbxdir[i] = 0.0;
+						if (fabs(rgbdir[i]) > fabs(bdir)) {
+							bdir = rgbdir[i];
+							bx = i;
+						}
+					}
+
+					/* See how close to the target we are */
+					terr = sqrt((tLab[1] - Lab[1]) * (tLab[1] - Lab[1])
+					          + (tLab[2] - Lab[2]) * (tLab[2] - Lab[2]));
+					if (terr < 0.1)
+						rgbdir[0] = rgbdir[1] = rgbdir[2] = 0.0;
+					rgbxdir[bx] = rgbdir[bx];
+
+			 		printf("%c%c Current x %.4f%c, y %.4f%c  DE %4.1f  R%c%c G%c%c B%c%c ",
+					       cr_char,
+					       ff == 0 ? '/' : '\\',
+					       Yxy[1],
+						   Yxy[1] > tYxy[1] ? '-': Yxy[1] < tYxy[1] ? '+' : '=',
+					       Yxy[2],
+						   Yxy[2] > tYxy[2] ? '-': Yxy[2] < tYxy[2] ? '+' : '=',
+					       icmCIE2K(tLab, Lab),
+					       rgbdir[0] < 0.0 ? '-' : rgbdir[0] > 0.0 ? '+' : '=',
+					       rgbxdir[0] < 0.0 ? '-' : rgbxdir[0] > 0.0 ? '+' : ' ',
+					       rgbdir[1] < 0.0 ? '-' : rgbdir[1] > 0.0 ? '+' : '=',
+					       rgbxdir[1] < 0.0 ? '-' : rgbxdir[1] > 0.0 ? '+' : ' ',
+					       rgbdir[2] < 0.0 ? '-' : rgbdir[2] > 0.0 ? '+' : '=',
+					       rgbxdir[2] < 0.0 ? '-' : rgbxdir[2] > 0.0 ? '+' : ' ');
+					fflush(stdout);
+				}
+				printf("\n");
+            }
 		}
 
 		/* Make sure drift comp. is set to the command line options */